Biotechnology in the 21st century presents the possibility of new solutions to food, environmental and health problems, with the ultimate object of promoting human prosperity. In recent years, the technology of using stem cells has been considered as a new way to treat incurable diseases. Previously, organ transplantation, gene therapy, etc., were presented for the treatment of incurable human diseases, but their efficient use has not been achieved due to immune rejection, short supply of organs, insufficient development of vectors, and an insufficient knowledge of disease genes.
With increasing interests in stem cell studies, it has been recognized that totipotent stem cells having the ability to form all the organs by proliferation and differentiation can not only treat most of diseases but also fundamentally heal organ injuries. Stem cells refer to cells having not only self-replication ability but also the ability to differentiate into at least two cells, and can be divided into totipotent stem cells, pluripotent stem cells, and multipotent stem cells. Many scientists have suggested clinical applicability of stem cells for the regeneration of all the organs and the treatment of incurable diseases, including Parkinson's disease, various cancers, diabetes and spinal damages.
Particularly, neural stem cells are capable of self-renewal and have the potential to differentiate into three major cell types of the central nerve system, including neurons, astrocytes, and oligodendrocytes. Accordingly, interests in neural stem cells are increasing recently, not only with regard to basic researches on mechanisms of proliferation and differentiation of stem cells and development of nervous systems, but also with regard to the possibility of new cell and gene therapy in neurological diseases, which are known not to be regulated once damaged, utilizing biological characteristics of the neural stem cells.
The concept that stem cells require specific cellular microenvironments, or niches, for their culture, is a well-established theory in stem cell biology. As techniques for selectively culturing neural stem cells, neurosphere formation, low-density culture, and high-density culture, etc., were reported, but it is known to be difficult to expand cells  in large-scale culture in an undifferentiated state.
Several researchers have attempted the large-scale culture of stem cells. However, human adult neural stem cells are particularly difficult to culture in vitro and also have limited ability to proliferate. For this reason, studies on human adult neural stem cells are at a standstill.
Accordingly the present inventors have conducted studies to overcome the problem of the limited ability of human adult neural stem cells to proliferate, and as a result, have found that, when primarily cultured adult neural stem cells are cultured after genes that can activate the signaling pathways in the neural stem cells have been transfected into the neural stem cell, the ability of the neural stem cells to proliferate is significantly increased, thereby completing the present invention.